A magnetic recording medium is generally obtained by applying on a non-magnetic support a magnetic coating composition, followed by drying. A magnetic coating composition is commonly prepared by dispersing uniformly in a binder resin ferromagnetic powder, an abrasive, an anti-static agent, a lubricant, a hardener, other additives, and optionally, a solvent.
It is a common technique to add carbon black to a magnetic layer that contains ferromagnetic metal powder or barium-ferrite tabular particles. By the addition of carbon black, the magnetic layer can have improved abrasion resistance and lower specific surface resistance. Addition of conductive powder (e.g. carbon black, graphite, silver powder, nickel powder) or a surfactant (natural, nonionic, anionic, cationic and ampholytic surfactants) to a magnetic layer to lower its specific surface resistance is known from Japanese Patent Examined Publication Nos. 22726/1971, 24881/1972, 26882/1972, 15440/1973, 26761/1973, U.S. Pat. Nos. 2271623, 2240472, 2288226, 2676112, 2676924, 2676975, 2691566, 2727860, 2730498, 2742379, 2739891, 3068101, 3158484, 3201253, 3210191, 3294540, 3415649, 3441413, 3442654, 3475174 and 3545974.
Addition of conductive powder has such a drawback that the packing density or dispersibility of barium-ferrite powder is lowered due to the presence of conductive powder (in the case of a floppy disc, conductive powder is added normally in an amount larger than that in a magnetic tape), which results in a lowered output and an increased noise level. Surfactants can lower the specific surface resistance of a magnetic layer, but not significantly. Moreover, if a large amount of a surfactant is contained, a magnetic layer inevitably has poor durability. Carbon black contained in a magnetic layer deteriorates the surface condition of the magnetic layer and lowers the packing density of a magnetic substance, and eventually affects adversely the electromagnetic conversion characteristics of a recording medium. In particular, carbon black with an average particle size of 50 m.mu. or more, which can allow a magnetic layer to have improved abrasion resistance, makes the surface of a magnetic layer excessively rough, thus causing a recording medium to have a lower chroma S/N.
Meanwhile, carbon black with an average particle size of 50 m.mu. or less and an oil absorption of 100 ml or more is used widely since it can lower the specific surface resistance of a magnetic layer. However, this carbon black cannot be dispersed well in a magnetic layer, and adversely affects the orientation and packing ratio of a magnetic substance, resulting in a lowered chroma output.
Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) Nos. 125927/1985, 296525/1986, 121927/1987 and 20488/1989 each disclose a recording medium in which a conductive layer containing carbon black is provided between a support and a magnetic layer.
In this recording medium, a conductive layer and a magnetic layer are formed on a support by the wet-on-dry method; i.e., a magnetic layer is formed on a conductive layer after the conductive layer is dried. One drawback of the wet-on-dry method is that a recording medium has poor durability because of poor adhesion between layers. This problem is more serious in a magnetic disc containing barium-ferrite magnetic powder. That is, in the case of a magnetic disc containing barium-ferrite magnetic powder, if layer formation is conducted by the wet-on-dry method, a magnetic layer tends to come off during operation due to its insufficient adhesion to other layers. Another drawback of the wet-on-dry method is that dust is likely to stick to the surface of a recording medium during running, resulting in frequent dropouts. Further, the wet-on-dry method is accompanied by such problem that the roughness of a dried surface of a conductive layer affects adversely the surface condition of a magnetic layer. A solvent contained in a magnetic coating composition is caused to diffuse to a conductive layer, causing a difficulty in smooth casting of the magnetic coating composition on the conductive layer.
Providing on a carbon black-containing subbing layer a 4.0 .mu.m or more-thick magnetic layer that contains ferromagnetic metal powder or barium-ferrite magnetic particles is already known in the art. However, such large thickness of a magnetic layer prevents carbon black from manifesting its effect of lowering the specific surface resistance of a magnetic layer. As a result, dust is likely to stick to a recording medium, causing frequent dropouts. In this technique, a magnetic layer is formed on a subbing layer by the wet-on-dry method which, as mentioned above, has such drawbacks as complicated procedures and deteriorated surface condition of a magnetic layer due to the roughness of a subbing layer. If the surface of a magnetic layer is too rough, a recording medium has a lower chroma S/N.
Under such circumstances, there has been an increasing demand for a magnetic recording medium with a greater memory capacity, a higher reproduction output and more improved durability.
However, none of conventional magnetic recording mediums, in particular, those for use as floppy discs, satisfy such demand.